Gated radiotherapy is known for cases where the affected area of a subject moves due to the respirations, cardiac beats, intestinal movements, and the like of a subject. In the gated radiotherapy, the three-dimensional position of a marker indwelling in the affected area or near the affected area is measured and kept track of by capturing fluoroscopic views of the subject body with X-rays from two directions at given time intervals, and the affected area is irradiated with the treatment beam when the affected area or the marker enters a predetermined region.
As ways to reduce the exposure of the subject to the fluoroscopic X-ray radiation, some approaches have been known. One is to reduce the frame rate of the fluoroscopy when it is known that the movement of the affected area in the body is slow. Another is to estimate the position of a current affected area based on the positions of the same affected area measured in the past, and to omit an upcoming fluoroscopic X-ray radiation if the error between the estimated position of the affected area and the actual affected area is small.
However, the former conventional technology is incapable of reducing the amount of exposure to the fluoroscopic radiation unless the movement of the affected area in the subject body is slow. The latter requires fluoroscopic radiation from a plurality of directions to estimate the current position of the affected area, so the amount of exposure remains high.